Residual pressure sensor and residual pressure sensor monitoring apparatus

ABSTRACT

A residual pressure sensor for verifying that a residual pressure is zero, and to a monitoring apparatus for monitoring for faults in the residual pressure sensor. In order to detect that a residual pressure has completely gone, the residual pressure sensor detects change conditions in the output therefrom, and an output for no residual pressure is not generated while there are changes. Moreover, an electrical power supply condition to a drive solenoid (21) of a solenoid valve for opening/closing a pressure supply line, is monitored using a fail-safe current sensor (21) which outputs a logic value &#34;0&#34; when the presence of a current is detected, and which outputs a logic value &#34;1&#34; when the presence of a current is not detected. Fault monitoring of the residual pressure sensor is then carried out based on the monitoring output conditions of the current sensor and the output conditions of the residual pressure sensor.

TECHNICAL FIELD

The present invention relates to a residual pressure sensor which uses aBourdon tube to detect a drop in pressure. In particular the inventionrelates to a residual pressure sensor which is suitable for guardsystems such as those covered under Chapter 9 of British Standard BS5304, for fail-safe verification of stoppage of drive power tohydraulically powered machinery. Moreover, the present invention relatesto a residual pressure sensor monitoring apparatus for monitoring if aresidual pressure sensor is normal or abnormal.

BACKGROUND ART

In cases for example wherein maintenance work is carried out with amovable section of a machine in a holding condition, or with operationswherein a movable section of the machine and an operator co-operatealternately, safety measures to protect the operator from accidents areextremely important. In the case of hydraulically powered machinery forexample, safety measures can be taken involving a method wherein anoperator is permitted to approach close to a movable section of amachine only after verifying that the pressure source has been cut offand that pressure is not being supplied to the movable section of themachine. In this case, a safety system can be considered wherein thepresence or absence of a pressure supply to the movable section of themachine is monitored for example by a residual pressure sensor, and theoperator is warned by the display of a lamp or the like.

As a pressure detection sensor suitable for such types of safetysystems, there is the residual pressure sensor previously proposed bythe present inventors in Japanese Unexamined Patent Publication No.6-307952.

This residual pressure sensor has a pressure sensing pipe (Bourdon tube)bent into an approximate C-shape, with a closed end, which is displacedin proportion to the introduced pressure, provided with a plate having aslit. Moreover, the construction involves a photocoupler comprisingmutually opposed light emitting and light receiving elements arrangedwith the plate therebetween, with an output from the photocoupler andclamped at a power source voltage and rectified by means of a voltagedoubler rectifying circuit.

In operation, the pressure sensing pipe is communicatively connected toa pressure supply pipe for supplying pressure from a pressure source toa movable section of a machine. In this condition, when there is nopressure in the pressure supply pipe so that the pressure introduced tothe pressure sensing pipe is practically zero, the closed end of thepressure sensing pipe is not displaced, and an alternating current lightbeam from a light emitting element which is driven by an alternatingcurrent signal from a signal generator, is received by the lightreceiving element via the slit of the plate. As a result, an alternatingcurrent output is generated from the photocoupler, so that an electricaloutput signal of a higher level condition (logic value "1") than thepower source voltage, is generated from the voltage doubler rectifyingcircuit, and a display lamp illuminated to indicate no pressure (movablesection of the machine stopped).

On the other hand, when there is a pressure supply to the movablesection of the machine so that the movable section of the machine is ina drive condition, pressure is also introduced into the pressure sensingpipe, displacing the closed end thereof in proportion to the introducedpressure. Due to this closed end displacement, the plate is alsodisplaced, shutting off the light beam projected from the light emittingelement towards the light receiving element. As a result, an alternatingcurrent output signal is not generated from the photocoupler, and hencethe output of a higher level than the power source voltage is notgenerated from the voltage doubler rectifying circuit (the rectifyingcircuit output becomes a low level condition of logic value "0"), sothat the display lamp does not come on, thus indicating a pressure(movable section of the machine movable).

With this residual pressure sensor, since at the time of a fault theoutput from the output terminal of the rectifying circuit is in the sameform (logic value "0") as that for a pressure (corresponding to a dangercondition), the construction is fail-safe.

However, with the abovementioned residual pressure sensor, when thepressure supply to the movable section of the machine is stopped, theclosed end of the pressure sensing pipe is gradually displaced with thedrop in pipe pressure. In this case, during the displacement of thepressure sensing pipe, the light from the light emitting element leaksthrough the slit of the plate on the tip of the pressure sensing pipewith movement of the plate, and at the point when the light receivedlevel of the light receiving elements exceeds a certain amount, anoutput for a high level condition of no pressure is generated from therectifying circuit. There is thus the problem that an output for noresidual pressure is produced before the residual pressure in thepressure sensing pipe is completely zero.

With a system for monitoring the residual pressure of a movable sectionof a machine using such a residual pressure sensor, then in the casewherein the pressure inlet for example of the pressure sensing pipe ofthe residual pressure sensor becomes blocked with foreign matter so thatpressure cannot be introduced thereto, then a display for no pressurecontinues. In this case there is thus the problem that a no pressuredisplay (safety), indicating safety to the operator, is given eventhough pressure is being supplied to the movable section of the machine.Accordingly, with such a system it is extremely important as a safetymeasure to monitor for faults in the residual pressure sensor.

As a pressure sensor using a Bourdon tube, there is a device previouslyproposed by Futsuhara and Sugimoto et. al. (Japanese Unexamined PatentPublication No. 6-288849).

With this pressure sensor, the construction is such that when pressuresupplied to a movable section of a machine from a pressure source, isintroduced to a pressure sensing pipe (Bourdon tube), the closed end ofthe pressure sensing pipe is displaced so that a member provided on thetip thereof is pressingly engaged with a free end of a cantilever springwhich is excited by an oscillator. The oscillation of the cantileverspring is thus stopped, stopping an output (giving a low levelcondition) from an oscillating element fixed to the cantilever spring,and hence warning of a pressure (movable section of the machinemovable). On the other hand, when the pressure supply to the movablesection of the machine is stopped so that the pressure inside thepressure sensing pipe drops, the closed end returns to the originalposition, releasing the engagement with the free end of the cantileverspring. The cantilever spring thus vibrates so that an output of a highlevel condition is generated by the oscillating element, indicating nopressure (movable section of the machine stopped).

With this pressure sensor also, there are problems similar to those forthe abovedescribed residual pressure sensor.

The present invention takes into consideration the abovementionedsituation with the object of a first aspect thereof, of providing aresidual pressure sensor which does not produce an output for noresidual pressure until the residual pressure has completely gone.Moreover, it is an object of a second aspect of the present invention toprovide a residual pressure sensor monitoring apparatus, in a systemincorporating a pressure supply control device for carrying out pressuresupply to a movable section of a machine at the time of electrical powersupply and for stopping pressure supply at the time of no electricalpower supply, which monitors if the residual pressure sensor is normalor abnormal by monitoring the power supply condition of the pressuresupply control device and the operating condition of the residualpressure sensor.

DISCLOSURE OF THE INVENTION

In view of the above, a residual pressure sensor according to a firstaspect of the invention, incorporating a pressure sensing pipe with oneend closed such that the closed end is displaced with anincrease/decrease in pressure introduced from another end opening, and apressure-electricity converter section which detects the displacementlocation of the closed end of the pressure sensing pipe and at the timeof a pressure increase, decreases an electrical output in accordancewith displacement of the closed end, and at the time of a pressuredecrease, increases an electrical output in accordance with displacementof the closed end comprises; an electrical output change detectiondevice for detecting whether or not the electrical output from thepressure-electricity converter section has a changing condition, andgenerating a low level output in the event of a changing condition, anda high level output in the event of a constant condition, and afail-safe first logical product operating device for carrying out alogical product operation on an output from the electrical output changedetection device and an output from the pressure-electricity convertersection, and generating an output of logic value "1" corresponding to ahigh level indicating no residual pressure, when both outputs are at ahigh level equal to or above a predetermined value, and generating anoutput of logic value "0" corresponding to a low level, at the time of afault.

In this way, while pressure remains in the pressure sensing pipe anddisplacement of the closed end is not stopped, an output for no residualpressure is not generated due to changes in the output from thepressure-electricity converter section, an output for no residualpressure not being generated until the residual pressure in the pipe hascompletely gone and the output from the pressure-electricity convertersection has become constant. Moreover, since at the time of a sensorfault the output form becomes one indicating danger, with residualpressure, then the construction is fail-safe.

The pressure-electricity converter section may comprise; a plate havinga slit and fixed to the pressure sensing pipe closed end so as to bedisplaced in accordance with displacement of the closed end, a lightsensor incorporating a light emitting element and a light receivingelement oppositely disposed with the plate therebetween, a first signalgenerator for supplying an alternating current signal to the lightemitting element of the light sensor to generate an alternating currentlight beam, and a first rectifying circuit for clamping at a powersource voltage and rectifying, an alternating current output from thelight sensor, the construction being such that when a pressure in thepressure sensing pipe is equal to or less than a predetermined pressure,a light beam from the light emitting element is received by the lightreceiving element via the slit.

In this case, instead of supplying an alternating current signal to alight emitting element, a method such as that of Futsuhara and Sugimotomay be used with a vibrating element fitted to the plate to vibrate theplate in a direction substantially perpendicular to a direction of lightemission from the light emitting element, so that the light emitted fromthe light emitting element is modulated to give alternating currentlight.

With this arrangement, it is possible to determine if the slit hasdropped off.

Moreover, a reflection type light sensor may be used if thepressure-electricity converter section comprises; a plate fixed to thepressure sensing pipe closed end so as to be displaced in accordancewith displacement of the closed end, a light sensor incorporating alight emitting element and light receiving element provided to one sideof the plate, a first signal generator for supplying an alternatingcurrent signal to the light emitting element of the light sensor togenerate an alternating current light beam, and a first rectifyingcircuit for clamping at a power source voltage and rectifying analternating current output from the light sensor, and the constructionis such that when a pressure in the pressure sensing pipe is equal to orless than a predetermined pressure, the light beam from the lightemitting element is reflected by the plate and received by the lightreceiving element.

Moreover, chattering of the output due to oscillation of the closed endof the pressure sensing pipe can be prevented if there is provided; twopressure-electricity converter sections which respectively generateelectrical output signals of a high level at the time of pressure levelsequal to or less than mutually different first and second pressurelevels, and a first self hold circuit with an output from thepressure-electricity converter section which generates an electricaloutput signal at pressure levels equal to or less than the firstpressure level, as a trigger input signal, and an output from thepressure-electricity converter section which generates an electricaloutput signal at pressure levels equal to or less than the secondpressure level which is higher than the first pressure level, as a resetinput signal, and which self holds the trigger input signal.

The electrical output change detection device may comprise; a secondsignal generating device for superimposing a high frequency alternatingcurrent signal on an output from the pressure-electricity convertersection, an amplifying device into which is input by way of a couplingcapacitor, the output from the pressure-electricity converter section onwhich is superimposed the high frequency alternating current signal ofthe second signal generating device, and wherein the amplified output issaturated when the output from the pressure-electricity convertersection is in a changing condition, and a second rectifying circuit forclamping the alternating current amplified output from the amplifyingdevice at the power source voltage and rectifying, the constructionbeing such that the rectified output from the second rectifying circuitis output to the first logical product operating device.

Moreover, the first logical product operating device may be constructedof a fail-safe window comparator having two input terminals, whichgenerates an alternating current output higher than the power sourcevoltage when each of the input signals input to the respective inputterminals are equal to or above a previously set lower limit thresholdvalue, and which generates an output of logic value "0" at the time of afault.

Furthermore with a residual pressure sensor monitoring apparatusaccording to a second aspect of the present invention, applicable to asystem incorporating a pressure supply control device for carrying outpressure supply to a movable section of a machine at the time ofelectrical power supply and for stopping pressure supply at the time ofno electrical power supply, which monitors that there is no pressuresupply to the movable section of the machine using a residual pressuresensor incorporating, a pressure sensing pipe with one end closed suchthat the closed end is displaced with an increase/decrease in pressureintroduced from another end opening, and a pressure-electricityconverter section which detects the displacement location of the closedend of the pressure sensing pipe and at the time of a pressure increase,decreases an electrical output in accordance with displacement of theclosed end, and at the time of a pressure decrease, increases anelectrical output in accordance with displacement of the closed end, theresidual pressure sensor monitoring apparatus being for monitoring ifthe operation condition of the residual pressure sensor is normal orabnormal, the residual pressure sensor monitoring apparatus may include;a current sensor which monitors the electrical power supply condition ofthe pressure supply control device, and generates a low level output oflogic value "0" at the time of electrical power supply, and generates ahigh level output of logic value "1" at the time of no electrical powersupply, and generates an output of logic value "0" at the time of afault, a fail-safe NOT operating device which carries out a NOToperation on the logical output from the residual pressure sensor, whichgenerates a low level output of logic value "0" at the time of supplypressure to the movable section of the machine, and a high level outputof logic value "1" at the time of no supply pressure, and generates anoutput of logic value "0" at the time of a fault, and which generates alow level output of logic value "0" at the time of a fault, and ajudgement device which judges if there is a residual pressure sensorfault based on respective logical outputs from the current sensor andthe NOT operating device, and when both logical outputs are logic value"0", generates a low level output of logic value "0" indicating a faultin the residual pressure sensor.

With such a construction, when an output of logic value "1" isgenerated, indicating that the residual pressure sensor has no residualpressure even though the pressure supply control device is switched onso that pressure is supplied to the movable section of the machine, theoutput from the judgement device becomes a logic value "0".Consequently, at the point when pressure supply is commenced, it can beknown that a blockage has occurred in the residual pressure sensor.

Moreover, the construction may be such that there is provided a secondlogical product operating device for carrying out a logical productoperation on the output from the judgement device and the output fromthe current sensor, and a third logical product operating device forcarrying out a logical product operation on the output from the secondlogical product operating device and the output from the residualpressure sensor, and the output from the third logical product operatingdevice is made the residual pressure sensor fault judgement output. Thenwhen during pressure supply, a blockage occurs in the pressure inlet ofthe residual pressure sensor, the output from the third logical productoperating device becomes a low level of logic value "0", so that theresidual pressure sensor fault can be known.

Moreover, the construction may be such that the judgement device is anadding circuit, and there is provided a fail-safe first windowcomparator with upper and lower threshold values set on either side ofan intermediate value of an addition output from the adding circuit,which generates a high level output of logic value "1" when an additionoutput within the threshold value range is input, and which generates anoutput of logic value "0" at the time of a fault, an off-delay devicewhich time delays a drop in the output from the first window comparatorto longer than a period from when the current sensor generates an outputindicating no current until the residual pressure sensor generates anoutput indicating no residual pressure, and a second window comparatorwhich generates an output of logic value "0" indicating a residualpressure sensor fault when the output from the off-delay device is lowerthan a predetermined level. Then if the residual pressure sensor has afault so that the output becomes a logic value "0", the output from thesecond window comparator becomes a low level of logic value "0" after anelapse of the delay time of the off-delay device, so that the residualpressure sensor fault can be known.

Moreover, the construction may be such that a counter is provided whichcounts a clock signal at the time of inputting an output from thecurrent sensor of a logic value of "1" indicating no current, and stopscounting at the time of inputting an output from the residual pressuresensor of logic value "1" indicating no pressure. It can then be knownthat when the counter value is excessively long, the period fromstopping the pressure supply until indication of no residual pressurehas become excessive due to deterioration of the pressure sensing pipecausing the return operation following the pressure stoppage to slowdown.

Moreover, the construction may be such that the residual pressure sensorcomprises; an electrical output change detection device for detectingwhether or not the electrical output from the pressure-electricityconverter section has a changing condition, and generating a low leveloutput in the event of a changing condition, and a high level output inthe event of a constant condition, and a fail-safe first logical productoperating device for carrying out a logical product operation on anoutput from the electrical output change detection device and an outputfrom the pressure-electricity converter section, and generating anoutput of logic value "1" corresponding to a high level indicating noresidual pressure, when both outputs are at a high level equal to orabove a predetermined value, and generating an output of logic value "0"corresponding to a low level, at the time of a fault.

Due to the above constructions, the residual pressure sensor does notgenerate a high level output of logic value "1" indicating no residualpressure until the residual pressure has completely gone. Therefore in asystem which is dangerous when the residual pressure has not completelygone even after stoppage of the pressure supply to the movable sectionof the machine, the safety of the operator can be more reliably ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the construction of a firstembodiment of a residual pressure sensor according to a first aspect ofthe present invention;

FIG. 2 is an electrical circuit diagram of the first embodiment;

FIG. 3 is a circuit diagram of a window comparator used in the firstembodiment;

FIG. 4(A) is an example of an amplifying circuit used in the firstembodiment, while

FIG. 4(B) is another example of an amplifying circuit;

FIG. 5 is a time chart for explaining an operation of the firstembodiment;

FIG. 6 is a structural diagram of the main components of a secondembodiment of a residual pressure sensor;

FIG. 7 is a structural diagram of the main components of a thirdembodiment of a residual pressure sensor;

FIG. 8 is a structural diagram of the main components of a fourthembodiment of a residual pressure sensor;

FIG. 9 is an electrical circuit diagram of a pressure-electricityconverter section of the fourth embodiment;

FIG. 10 is a time chart illustrating a relation between residualpressure and an output from the pressure-electricity converter sectionof the fourth embodiment;

FIG. 11 is a circuit diagram illustrating a first embodiment of aresidual pressure sensor monitoring apparatus according to a secondaspect of the present invention;

FIG. 12 is a basic circuit diagram of an adding connection section for acurrent sensor side output and a residual pressure sensor output of thefirst embodiment according to the second aspect;

FIG. 13 is a time chart for explaining an operation of the firstembodiment according to the second aspect;

FIG. 14 is a truth table illustrating a logical relation between alogical output related to a current flow condition, and a logical outputrelated to a pressure supply condition;

FIG. 15 is a circuit diagram of a second embodiment of a residualpressure sensor monitoring apparatus of the present invention;

FIG. 16 is a circuit diagram of a rectifying circuit in a self holdcircuit in the second embodiment;

FIG. 17 is a circuit diagram of a third embodiment of a residualpressure sensor monitoring apparatus of the present invention;

FIG. 18 is a circuit diagram of a fourth embodiment of a residualpressure sensor monitoring apparatus of the present invention; and

FIG. 19 is a time chart for explaining an operation of the fourthembodiment of the residual pressure sensor monitoring apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a description of embodiments of the present invention withreference to the drawings.

FIG. 1 is a structural diagram of a first embodiment of a residualpressure sensor according to a first aspect of the present invention.

In FIG. 1, a base 1 has a pressure inlet pipe 2 in the form of a hollowsquare block, secured to a lower portion thereof. The pressure inletpipe 2 is for connecting to and taking pressure from for example asupply pipe (not shown) for supplying pressure from a pressure source toa movable section of a machine. One end of the pressure inlet pipe 2 isclosed, while the other end is provided with a hollow cylindricalthreaded portion 3 for connecting to the supply pipe, with a pressureinlet 4 for taking the pressure when connected. A pressure sensing pipe5 bent into an approximate C-shape, has a base end communicativelyconnected and secured to the pressure inlet pipe 2, and a tip end formedas a closed end 5A. The construction is such that when pressure isintroduced via the pressure inlet pipe 2, the closed end 5A is displacedin the direction of the arrow in FIG. 1 (upwards direction) withincrease in pressure.

A plate 6 having a slit 7, is attached to the tip of the closed end 5Aof the pressure sensing pipe 5, in approximate parallel with thedirection of displacement of the closed end 5A. Moreover, a photocoupler8 serving as a light sensor, having a light emitting element 8a and alight receiving element 8b opposed to each other with the plate 6therebetween, is secured to the base 1.

As shown in FIG. 2, the light emitting element 8a of the photocoupler 8,is activated by an alternating current signal supplied from a firstsignal generator 9 via a current reducing resistor R1, to produce analternating current light beam. The light receiving element 8b, has apower source voltage Vcc input for example via a resistor R2 to thecollector, and generates an electrical output signal from a point Z1 inFIG. 2, in proportion to the amount of light received. The signalreceived by the light receiving element 8b is rectified by a voltagedoubler rectifying circuit 10 serving as a first rectifying circuit,comprising capacitors C1, C2 and diodes D1, D2. Here the plate 6, thephotocoupler 8, the first signal generator 9 and the voltage doublerrectifying circuit 10 constitute a pressure-electricity convertersection.

An output terminal Z2 of the voltage doubler rectifying circuit 10 isconnected via a resistor R3 to one input terminal A of a fail-safe twoinput window comparator WC1, The window comparator WC1 corresponds to afirst logical product operating device.

The fail-safe two input window comparator WC1 is disclosed for examplein U.S. Pat. No. 4,661,880, and in papers such as that of M. Kato, M.Sakai, K. Futsuhara, etc. entitled "LSI Implementation and SafetyVerification of Window Comparation Used in Fail-Safe Multiple-ValuedLogic Operation" (IEICE TRANS. ELECTRON., VOL. E76-C, NO. 3, MARCH1993).

The two input window comparator WC1 as shown for example in FIG. 3,comprises resistors R11˜R28 and transistors Q1˜Q7. Input terminals A, Bhave threshold values with respective upper and lower limits. When asignal having an input level within the respective threshold valueranges is input to each of the input terminals A, B,oscillation at ahigh frequency occurs to produce an alternating current output signal.

That is to say, the construction is such that with respective inputvoltages of the input terminals A, B as V1, V2 and the power sourcevoltage as Vcc, oscillation only occurs when the respective inputsignals satisfy the following conditions; ##EQU1##

A second signal generator 11 is for generating an alternating currentsignal. This alternating current signal is superimposed on an outputsignal e₁ from the voltage doubler rectifying circuit 10, by way of acapacitor C3 and a resistor R4. The first signal generator 9 may be usedfor the second signal generator 11.

An amplifier 12 amplifies the output signal e₁ from the voltage doublerrectifying circuit 10 which has been superimposed with the alternatingcurrent signal, and which is input by way of a coupling capacitor C4.The amplifier 12 and the capacitor C4 together constitute a widebandamplifying circuit. It also amplifies the slow fluctuations occurring inthe output signal e₁ from the voltage doubler rectifying circuit 10accompanying the output frequency ω of the second signal generator 11,and the pressure changes (displacement of the pressure sensing pipe 5).

The amplifier 12 as shown for example in FIG. 4(A), is a known amplifiercomprising an emitter ground amplifying circuit and an emitter followeramplifying circuit, having resistors R31˜R35 and capacitors Q11, 012. Anamplifier 12' as shown in FIG. 4(B), comprising a FET amplifying circuitand an emitter follower amplifying circuit, having resistors R36˜R40, afield effect transistor (FET) Q13 and a transistor Q14, may also beused.

A voltage doubler rectifying circuit 13 serving as a second rectifyingcircuit, comprises two capacitors C5, C6 and two diodes D3, D4. Anoutput signal e₂ from the amplifier 12 is clamped at the power sourcevoltage Vcc. The capacitors C5, C6 are set so as to rectify thealternating current signal of output frequency ω from the second signalgenerator 11, and generate a direct current output signal e₃, with theslow fluctuation signal (output signal e₁ from the voltage doublerrectifying circuit 10) accompanying the pressure changes being blockedby the coupling capacitor C5. Consequently, the voltage doublerrectifying circuit 13 has a rectifying function, and a filter functionwhich passes high frequency signals. The output signal e₃ from thevoltage doubler rectifying circuit 13 is input to the other inputterminal B of the window comparator WC1. Here the second signalgenerator 11, the amplifier 12 and the voltage doubler rectifyingcircuit 13 constitute the electrical output change detection device.

A voltage doubler rectifying circuit 14 has a similar construction tothe voltage doubler rectifying circuit 10. It clamps the oscillatingoutput signal from the window comparator WC1 at the power source voltageVcc, to generate an output signal e₄.

Here, with the upper limit threshold value of the input terminal A ofthe window comparator WC1 as Th1AH, and the lower limit threshold valueas Th1AL, and the upper limit threshold value of the input terminal B asTh1BH, and the lower limit threshold value as Th1BL, the windowcomparator WC1 carries out the following threshold value operation,depending on the input levels of the respective input signals e₁ and e₃.##EQU2##

Here F is the logical product output from the window comparator WC1,F_(A) is the logical input signal input to the input terminal A, andF_(B) is the logical input signal input to the input terminal B. Symbol"•" indicates the logical product.

Accordingly, with the voltage doubler rectifying circuit 14, when thelogical binary output is F1 so that the window comparator WC1 oscillates(when F=1), the output signal e₄ is generated as a logic value "1"output (F1=1) of a higher level than the power source voltage Vcc, whilewhen the window comparator WC1 does not oscillate (when F=0), resultingin a low level (power source voltage Vcc), the output signal e₄ isgenerated as logic value "0" output (F1=0).

The operation of the circuit of the first embodiment will now bedescribed with reference to the time chart of FIG. 5.

In the pressurized condition with pressure supplied to the movablesection of the machine, the light beam from the light emitting element8a to the light receiving element 8b is cut off by the plate 6 due todisplacement of the pressure sensing pipe 5, so that the output from thephotocoupler 8 disappears, and the level of the output signal e₁ fromthe voltage doubler rectifying circuit 10 becomes a low level (powersource voltage Vcc). When in this condition, the pressure supply to themovable section of the machine is stopped, a non-pressurized conditionresults with the pressure sensing pipe 5 being gradually displaced toits original position with the drop in pressure therein. During thecourse of this displacement, the light beam amount received by the lightreceiving element 8b by way of the slit 7 gradually increases withmovement of the plate 6, so that the level of the output signal e₁ fromthe voltage doubler rectifying circuit 10 also slowly increasesaccordingly, becoming a constant value at the point when the pressureinside the pressure sensing pipe 5 is completely gone. The output signalof the second signal generator 11 is voltage divided by the resistor R3and the resistor R4 and superimposed on this output signal e₁ from thevoltage doubler rectifying circuit 10, and input to the amplifier 12 byway of the capacitor C4. With this superimposed signal as e₀, thecondition of the input signal to the amplifier 12 is shown in FIG. 5(a).In FIG. 5, t₁ is the time when the light beam first passes through theslit 7, while t₃ is the time when the pressure inside the pressuresensing pipe 5 is completely gone.

With the amplifier 12, when the direct current component of the inputsignal (voltage doubler rectifying circuit output signal e₁) is constantrelative to an operating point V_(B), the alternating current componentsignal e₀ is amplified and output (the amplified output at this time isshown as Ge₀, where G is the gain). Moreover, when the direct currentcomponent of the input signal is changing, this is amplified and reachesthe saturation level, and the alternating current amplified signal Ge₀disappears.

Accordingly, while the output from the amplifier 12 is saturated,accompanying the changes in the output signal e₁ from the voltagedoubler rectifying circuit 10 (from time t1' to time t4), then as shownin FIG. 5(b), the alternating current signal component of the signalgenerator 11 in the output from the amplifier 12 disappears. Moreover,even after the output signal e₁ from the voltage doubler rectifyingcircuit 10 has become constant, the alternating current signal componentGe₀ does not appear during the time determined by a time constant τ ofthe coupling capacitor C4 and the input resistance of the amplifier 12,but appears from the point when this time has elapsed. Also at the timeof change from the non-pressurized condition to the pressurzedcondition, the alternating current signal component of the signalgenerator 11 similarly disappears during the time t5˜t6.

With the voltage doubler rectifying circuit 13, the electrostaticcapacity of the capacitors C5, C6 is set so as to rectify thealternating current signal of frequency ω from the second signalgenerator 11. Therefore, in the amplified output periods (t1'˜t4 andt5˜t6) as shown in FIG. 5(c), wherein there is no alternating currentsignal component Ge₀, the output signal e₃ from the voltage doublerrectifying circuit 13 becomes a low level (power source voltage Vcc).

Accordingly, while changing from the pressurized condition to thenon-pressurized condition, then during reduction of the pressure insidethe pressure sensing pipe 5 (time t₂ in FIG. 5(a)), the output level ofthe voltage doubler rectifying circuit 10 becomes equal to above thelower limit threshold value Th1AL of the input terminal A of the windowcomparator WC1. However, at this time, the output signal e₃ from thevoltage doubler rectifying circuit 13 is at a lower level than the lowerlimit threshold value Th1BL of the input terminal B of the windowcomparator WC1, so that an output signal is not produced from the windowcomparator WC1. After this, and a little after the residual pressureinside the pressure sensing pipe 5 becomes completely zero (outputsignal e₁ constant), shown as time t4' in FIG. 5(c), the output from thevoltage doubler rectifying circuit 13 becomes equal to or above thelower limit threshold value Th1BL of the input terminal B, so that anoscillating output from the window comparator WC1 is generated, causingthe output signal e₄ from the voltage doubler rectifying circuit 14 tobecome a high level (F1=1), and this point there is an output indicatingno residual pressure. That is to say, the high level output signal F1=1always has a time delay δ from after the residual pressure becomes zero.

Moreover, at the time of changing from the non-pressurized condition tothe pressurized condition (time t₅ in FIG. 5), the level of the outputsignal e₁ from the voltage doubler rectifying circuit 10 immediatelybecomes less than the lower limit threshold value Th1AL of the inputterminal A of the window comparator WC1. Therefore the output signal e₄from the voltage doubler rectifying circuit 14 becomes a low level(F1=0), with the output indicating a pressure.

With the circuit construction of the embodiment as described above, thesignal of F1=1 indicating zero residual pressure is not generated fromthe voltage doubler rectifying circuit 14 until the displacement of theclosed end 5A of the pressure sensing pipe 5 has completely stopped;that is to say the residual pressure is completely at zero. Therefore,from the point of ensuring operator safety, the arrangement is farsuperior to the conventional arrangement wherein the output for noresidual pressure is generated prior to the pressure being completelygone.

Moreover, with the circuit of FIG. 2, the voltage doubler rectifyingcircuits 10, 13, and 14 are constructed such that at the time of afault, a high level output signal (indicating safety) is never produced.Furthermore, the wide band amplifier constituted by the capacitor C4 andthe amplifier 12 is also a circuit which does not generate analternating current output signal at the time of a fault. For example,if a short circuit fault occurs in the capacitor C4, a power sourcevoltage Vcc is directly applied to the input side of the amplifier 12 byway of the diodes D1, D2 of the voltage doubler rectifying circuit 10and the resistor R3, so that the signal e₀ cannot be amplified andoutput. Moreover, if a fault occurs in the window comparator WC1, anoscillating output signal is not produced. Accordingly, the circuit ofthe present embodiment is a fail-safe circuit which does not produce anoutput signal of logic value "1" indicating no residual pressure(safety) at the time of a fault.

The upper limit threshold value of the window comparator WC1 is notalways necessary. Accordingly, the upper limit threshold value may beset to a sufficiently high level.

In the case wherein a transmission type light sensor is used in theabovedescribed form wherein at the time of no pressure, light isreceived by the light receiving element 8b via the slit 7 to thus detectno pressure, then if the plate 6 drops off from the pressure sensingpipe closed end 5A, the emitted light from the light emitting element 8ais always received by the light receiving element 8b, resulting in adangerous situation with safety being indicated.

Therefore, when a transmission type light sensor is used, the lightsource can be a direct current light, with a construction according to asecond embodiment shown in FIG. 6, wherein an oscillator 81 is fitted tothe plate 6. The oscillator 81 is driven by a signal generator 82 so asto continually excite the plate 6 at a higher frequency than the lightbeam of the light emitting element 8a, and in a direction substantiallyperpendicular to the light emission direction of the light emittingelement 8a, thereby modulating the light beam from the light emittingelement 8a. In this case, the time constant of the rectifying circuit 10is set to conform to the oscillation frequency of the oscillator 81.

With such a construction, only when a light beam modulated by theoscillations of the plate 6 is received by the light receiving element8b is the charging/discharging of the capacitors C1, C2 of therectifying circuit 10 carried out and a high level electrical outputsignal produced at the output terminal Z2 of the rectifying circuit 10.If the plate 6 drops off so that modulation ceases, then even though thelight from the light emitting element 8a is transmitted to the lightreceiving element 8b, a high voltage above the power source voltage Vccis not produced at the output terminal Z2 of the rectifying circuit 10,so that an erroneous high level output indicating no pressure in spiteof there being a pressure, is not produced.

The above embodiments both illustrate a transmission type light sensor.However it is also possible to use a reflection type light sensoraccording to a third embodiment as shown in FIG. 7, wherein a plate 6'having no slit is fixed to the closed end 5A of the pressure sensingpipe 5, and a photocoupler 8' is arranged with a light emitting elementand light receiving element provided to one side of the plate 6'.

With such a construction, when there is a pressure (movable conditionfor the movable section of the machine), the plate 6' is raised withdisplacement of the closed end 5A, so that the light from the lightemitting element is not reflected by the plate 6', and is thus nottransmitted to the light receiving element, resulting in a low leveloutput form indicating danger. On the other hand, when there is nopressure (stop condition for the movable section of the machine),displacement of the pressure sensing pipe 5 ceases so that the lightfrom the light emitting element is reflected by the plate 6', andtransmitted to the light receiving element. As a result, a high levelelectrical output signal indicating safety is generated from therectifying circuit 10.

A fourth embodiment of a residual pressure sensor will now be describedwith reference to FIGS. 8 to 10. Parts similar to those of theabovementioned embodiments are indicated by the same symbols, anddescription is omitted.

With this embodiment as shown in FIG. 8, two slits 92A, 92B are providedin a plate 91 fixed to the closed end 5A of the pressure sensing pipe 5,and two photocouplers 8A, 8B of the same construction as thephotocoupler 8 in FIG. 2, are provided opposite to the respective slits92A, 92B. The slit 92B has a wider slit width than that of the slit 92A,such that the positional relationship becomes as shown in FIG. 8 whenthe residual pressure is zero, as shown by the chain line in FIG. 8 (theposition wherein the light beam is received by the light receivingelement. This is different from the position wherein the residualpressure is completely zero). The construction is thus such that, withthe introduction of pressure, and displacement of the closed end 5A ofthe pressure sensing pipe 5, the light of the photocoupler 8A oppositeto the slit 92A is first cut off by the plate 91, after which the lightof the photocoupler 8B opposite to the slit 92B is cut off.

Moreover, as shown in FIG. 9, outputs X_(A), X_(B) of the respectivephotocouplers 8A, 8B, which have been rectified by the rectifyingcircuit 10, are input to a first self hold circuit 96 comprising, a wellknown fail-safe AND gate 93 which does not generate an output at thetime of a fault, a rectifying circuit 94, and a resistor 95. The selfhold circuit 96 is constructed with the output from the photocoupler 8Aas a trigger input, the output from the photocoupler 8B as a resetinput, and with the trigger input self held by an output y.

Next is a description of the operation of the circuit with reference tothe time chart of FIG. 10.

When the residual pressure is zero, the light receiving elements 8b ofthe photocouplers 8A, 8B receive light via the slits 92A, 92B, so thatboth inputs to the AND gate 93 of the self hold circuit 96 are a highlevel, and the output signal y of the self hold circuit 96 becomes ahigh level output of logic value "1" indicating safety.

When pressure is supplied from this condition, the closed end 5A of thepressure sensing pipe 5 is displaced with an increase in pressure, sothat the plate 91 is also displaced in an upward direction in FIG. 8.Then, when the pressure becomes higher than the level shown at P_(A) inFIG. 10, at first the slit 92A moves outside of the light path of thelight emitting element 8a of the photocoupler 8A so that the light beamis shut off. The rectified output X_(A) from the photocoupler 8A thusceases, so that the trigger input to the self hold circuit 96 stops.However, since the trigger input is self held by the rectified outputfrom the rectifying circuit 94, of the output y of the self hold circuit96, then the output signal y of the self hold circuit 96 is held at alogic value "1 ". When after this the pressure rises further to becomehigher than P_(B), the light from the photocoupler 8B of the slit 92B isalso shut off, so that the rectified output X_(B) stops. As a result,the output signal of the self hold circuit 96 becomes a low level oflogic value "0", warning that the movable section of the machine is in amovable condition.

After this, when the pressure supply to the movable section of themachine is stopped, the plate 91 moves in the opposite direction due tothe pressure stoppage, and when the residual pressure falls below P_(B),light is received by the light receiving element 8b of the photocoupler8B by way of the slit 92B, and the rectified output X_(B) of thephotocoupler 8B is input to the AND gate 93 of the self hold circuit 96.At this time, since the rectified output X_(A) of the photocoupler 8A isnot yet input, then the output signal y of the self hold circuit 96remains at logic value "0".

The pressure then falls further and when the residual pressure fallsbelow P_(A), light is received by the light receiving element 8b of thephotocoupler 8A by way of the slit 92A, and the rectified output X_(A)of the photocoupler 8A is also input to the AND gate 93 of the self holdcircuit 96. As a result, the self hold circuit 96 is triggered and theoutput signal y of the self hold circuit 96 becomes a high level oflogic value "1".

Hence, with the fourth embodiment at the time of a pressure rise, whenthe pressure becomes higher than P_(B), the output signal y of the selfhold circuit 96 becomes a logic value "0", while at the time of a dropin the residual pressure, when the pressure falls below P_(A), theoutput signal y of the self hold circuit 96 becomes a logic value "1".Consequently, a hysteresis width T as shown in FIG. 10 is obtained,enabling prevention of a chattering phenomena wherein the output signaly of the self hold circuit 96 is switched ON and OFF due to oscillationof the plate 91 with fluctuations in pressure.

As follows is a description of a residual pressure sensor monitoringapparatus according to a second aspect of the present invention.

FIG. 11 shows a first embodiment of the residual pressure sensormonitoring apparatus according to the second aspect, in the form of acircuit applied to a system wherein pressure supply to a movable sectionof a machine is carried out with a solenoid valve.

In FIG. 11, a solenoid 21 is for driving a solenoid valve (not shown),disposed for example in a pressure supply pipe. The solenoid valve,serving as a pressure supply control device, is switched on (opened)with supply of a current I, to thus open a pressure supply pipeconnected to a movable section of a machine to supply pressure thereto.A current sensor 22 monitors the current I flowing in the solenoid 21,and is constructed such that an output signal e₅ therefrom becomes a lowlevel when the current I flows in the solenoid 21, and becomes a highlevel when there is no current I.

The current sensor 22 comprises; a saturable magnetic ring core 23 woundwith three windings, namely first to third windings N1˜N3, a signalgenerator 24 for supplying a high frequency signal to the first windingN1, an AC amplifier 25, for example of the same construction as theamplifier 12 of FIG. 2, connected to the second winding N2 foramplifying a signal therefrom, and a voltage doubler rectifying circuit26 serving as a third rectifying circuit, for rectifying an output fromthe AC amplifier 25. The third winding N3 is connected in series to apower lead of the solenoid 21.

With this current sensor 22, when the solenoid drive current I does notflow in the third winding N3, the high frequency signal supplied to thefirst winding N1 from the signal generator 24 by way of a resistor R50is transmitted to the second winding N2 by way of the ring core 23, andthe received signal output then amplified by the AC amplifier 25. On theother hand, when the current I flows in the third winding N3, the ringcore 23 becomes saturated. As a result, transmission of the signal fromthe first winding N1 to the second winding N2 is impaired, and theamplified output from the AC amplifier 25 drops considerably.Consequently, the output signal e₅ clamped at the power source voltageVcc by the voltage doubler rectifying circuit 26 and rectified, with alogical output thereof as F2, becomes a logic value "1" (F2=1) of a highlevel (higher than power source voltage Vcc) when there is no current I,and a logic value "0" (F2=0) of a low level (e₅ =˜Vcc) when the currentI flows.

The current sensor 22, has a fail-safe construction in that a signal isnot produced in the winding N2, when the signal generator 24 is faulty,or a disconnection fault occurs in the resistor R50, or a disconnectionfault occurs in the windings N1, N2. Such a current sensor 22 isdisclosed for example in the paper of M. Kato, K. Futsuhara, and M.Mukaidono, entitled "Construction of Magnetic Sensors for AssuringSafety" (Proc. of 2nd International conf. on Human Aspects of AdvancedManufacturing and Hybrid Automation, Honolulu (August 1990)).

A window comparator WC2 is a fail-safe device of the same constructionas the beforementioned window comparator WC1, with the output signal e₄(indicating the presence or absence of residual pressure) from theresidual pressure sensor of FIG. 2 input commonly to the input terminalsA, B. The power source voltage Vs of the window comparator WC2 howeveris set to be lower than the power source voltage Vcc of the windowcomparator WC1 (Vs<Vcc). With the window comparator of the circuitconfiguration of FIG. 3, the respective threshold values of the upperand lower limits are determined in proportion to the power sourcevoltage. Consequently, setting the power source voltage Vs of the windowcomparator WC2 lower than the power source voltage Vcc of the windowcomparator WC1, enables the level (Vcc) of logic value "0" (F1=0) of theoutput signal e₄ from the window comparator WC1 to be set within theupper and lower limit threshold value range of the window comparatorWC2. As a result, a NOT operation on the logical output F1 from theresidual pressure sensor can be executed by the window comparator WC2.Moreover, an upper limit threshold value Th2H and a lower limitthreshold value Th2L of the window comparator WC2 are set on either sideof the low level Vcc (F1=0) of the output signal e₄ from the windowcomparator WC1. Consequently, the window comparator WC2 generates anoutput of logic value "0" when the input signal (F1) is logic value "1", and generates an output of logic value "1" when the input signal islogic value "0", thus corresponding to a NOT operating device with thelogical relation of the input and output becoming a negative relation(F1=F1).

A voltage doubler rectifying circuit 27 which serves as a judgementdevice, has a similar construction to the voltage doubler rectifyingcircuit 10. The construction however is such that the output signal ofthe window comparator WC2 is clamped at the output signal e₅ from thevoltage doubler rectifying circuit 26. FIG. 12 shows the connections forthe voltage doubler rectifying circuit 26 and the voltage doublerrectifying circuit 27. In FIG. 12, symbols C7˜C10 denote capacitorswhile symbols D5˜D8 denote diodes. If a logical output signal of theoutput signal e₆ from the voltage doubler rectifying circuit 27 is F3,then when the logical outputs of the current sensor 22 and the windowcomparator WC2 are both "0", then F3=0, and when the logical output ofone or the other is "1", then F3=1, while when the logical outputs ofboth are "1", then F3=2. The voltage doubler rectifying circuit 27 thushas the function of an adding circuit which carries out an logicaladdition operation on the respective logical outputs of the currentsensor 22 and the window comparator WC2,

Next is a description of the operation, with reference to the timecharts of FIG. 13.

In FIG. 13, time chart (a) shows the output signal e₅ from the currentsensor 22. When the current I flows in the solenoid 21 of the solenoidvalve (when the solenoid valve is ON), then the output signal becomes e₅≈Vcc (logic level F2=0), while when the current I does not flow (whenthe solenoid valve is OFF), the output signal becomes e₅ ≈Vcc+V1 (logiclevel F2=1). Symbol V1 denotes the output voltage of the current sensor22 when there is no current I. Time chart (b) shows the output signal e₄from the residual pressure sensor (the output from the voltage doublerrectifying circuit 14 of FIG. 2). In the pressurized condition, e₄ =Vcc(logic level F1=0), while in the condition with no residual pressure, e₄=Vcc+V2 (logic level F1=1). Symbol V2 denotes the output voltage of theresidual pressure sensor when there is no residual pressure. Time chart(c) shows the logic level of the output signal e₆ from the voltagedoubler rectifying circuit 27, which rectifies the output signal of thewindow comparator WC2. Here, V3 is the output voltage of the windowcomparator WC2.

When the solenoid valve is switched ON to give the pressurizedcondition, the current I flows giving a logical output F2=0, with theoutput signal e₅ from the current sensor 22 at a low level (e₅ ≈Vcc). Atthis time, the output signal e₄ from the residual pressure sensorbecomes F1=0 with a low level (e₄ =Vcc) indicating a pressure, and sinceas shown in FIG. 13(b), the threshold value range of the windowcomparator WC2 is set on either side of the power source voltage Vcc,then the window comparator WC2 gives a logical output F1=1 (here thesymbol " " indicates the logical negation). Consequently, in thepressurized condition, the output signal e₆ from the voltage doublerrectifying circuit 27 becomes e₆ =Vcc+V3, and the logical value of thelogical output F3 becomes F3=1. When at time T1 (corresponding to timet1 in FIG. 5), the solenoid valve is switched OFF, the output signal e₅from the current sensor 22 becomes e₅ =V1+Vcc (logical value of F2=1).However an interval T_(D) is required until the pressure dropssufficiently for the residual pressure sensor to indicate no residualpressure, and at time T2 (corresponding to time t4 in FIG. 5), theoutput signal e₄ from the residual pressure sensor becomes e₄ =V2+Vcc(logic value of F1=1). The window comparator WC2 thus generates anoscillating output until time T2. Consequently, with the output signale₆ from the voltage doubler rectifying circuit 27, e₆ =Vcc+V3 (logicalvalue of F3=1) up until time T1, while between times T1 and T2, e₆=Vcc+V1+V3 (logical value of F3=2).

Here, since the logical output F3=2is produced when F1=1 and F2=1, thenthis expresses the logical product output of logical outputs F1 and F2.

From time T2 onwards, the output signal e₄ from the residual pressuresensor becomes e₄ =Vcc+V2, and since this exceeds the upper limitthreshold value Th2H of the window comparator WC2, then the outputsignal from the window comparator WC2 becomes a low level, and theoutput signal e₆ from the voltage doubler rectifying circuit 27 becomese₆ =Vcc+V1 (F3=1). Consequently, when the residual pressure sensor isoperating normally, then as shown in FIG. 13(c), the logical output F3from the voltage doubler rectifying circuit 27 is always F3≧1.

Now, if prior to supplying pressure to the movable section of themachine, a blockage occurs in the pressure inlet 4 of the residualpressure sensor, then the pressure does not flow into the pressuresensing pipe 5, so that the logical output F1 from the residual pressuresensor is always F1=1 (e₄ =Vcc+V2), and as shown in FIG. 13(d) the upperlimit threshold value Th2H of the window comparator WC2 is exceeded, sothat an oscillating output is not generated in the window comparatorWC2. Therefore, when the solenoid valve is switched ON to pressurize,then as shown in FIG. 13(e), the logical output becomes F3=0 with theoutput signal e₆ from the voltage doubler rectifying circuit 27 at e₆=Vcc. When the solenoid valve is switched OFF, the logical outputbecomes F3=1 due to the output from the current sensor 22.

Hence with the circuit of FIG. 11, in the case of a blockage in theresidual pressure sensor at the point when the solenoid valve isswitched ON to pressurize, then an output corresponding to a logic valueof F3=0 is produced. It can thus be known if there is a blockage in theresidual pressure sensor, by judging if an output corresponding to alogic value of F3=0 has been produced in the logical output F3 from thevoltage doubler rectifying circuit 27. Consequently, with systems usingpressure, it can be verified at the time of system operation, whether ornot a residual pressure sensor is able to indicate danger, and hence ifsubsequent operations can be carried out. The safety of systems usingpressure can thus be improved.

The logical structure of FIG. 11 can be expressed by the followingequation:

    F3=F2+F1                                                   (4)

where symbol "+" indicates addition.

Since the logical variable F2 expresses the presence or absence of thecurrent I of the solenoid 21, then expressing the presence of current Ias "1" and the absence as "0" gives:

    F2=I                                                       (5)

Equation (4) can thus be expressed as:

    F3=I+F1                                                    (6)

FIG. 14 shows a truth table for the logical outputs I and F1.

The combination of I=0 and F1=1 giving F3=1, indicates the detection ofa pressure when a current flows in the solenoid 21, while thecombination of I=1, F1=0 giving F3=1, indicates the non detection of apressure when a current does not flow in the solenoid 21, bothindicating that the residual pressure sensor is operating normally. Thecombination of I=1, F1=1giving F3=2, indicates that the operator hasswitched the solenoid valve OFF but a pressure still remains, being anormal operation of the circuit.

The combination of I=0 and F1=0 giving F3=0, indicates that the residualpressure sensor cannot show a pressurized condition in spite of acurrent flowing in the solenoid 21. Such a phenomena is produced byblockage of the pressure inlet 4 of the residual pressure sensor.

Accordingly, if the current sensor and the residual pressure sensor arenormal, then F3≦1 indicates normal operation of residual pressuredetection. This normal operation can be displayed for example using adisplay diode D9 as shown by the dotted line of FIG. 12.

With the circuit of FIG. 11, in the case wherein a blockage occursduring an established pressurised condition, then since an output (F1=0)for residual pressure is generated even after switching OFF the solenoidvalve, the output F3=0 does not eventuate, resulting in a dangeroussituation wherein a logical output indicating a normal residual pressuresensor is generated. Consequently, with a system wherein there is dangerif the residual pressure has not become completely zero even though theoperator has switched off the solenoid, it is necessary to verify thatthe logical output from the residual pressure sensor has become F1=1 (noresidual pressure).

FIG. 15 is a circuit to solve this problem, being a circuit of a secondembodiment which verifies, after switching off the solenoid valve, thatthere is no blockage in the residual pressure sensor, and generates averification output for no residual pressure.

In FIG. 15, a window comparator WC3 has a similar construction to thewindow comparator WC1 of FIG. 2, and constitutes, together with arectifying circuit 31, a self hold circuit 30 serving as a secondlogical product operating device, with a logical output F3 (output fromthe voltage doubler rectifying circuit 27 of FIG. 11) input to an inputterminal A as a trigger input, and a logical output F2 (output from thecurrent sensor 22) input to an input terminal B as a reset input. Anupper limit threshold value Th3AH and a lower limit threshold valueTh3AL of the input terminal A, are set as shown in FIG. 13(c), on eitherside of a logic level of logical output F3=2 (e₆ =Vcc+V1+V3). An upperlimit threshold value Th3BH and a lower limit threshold value Th3BL ofthe input terminal B are set as shown in FIG. 13(a), on either side of alogic level of logical output F2=1 (e₅ =Vcc+V1).

As shown in FIG. 16, the rectifying circuit 31 comprises capacitorsC11˜C14 and diodes D11˜D14, and is constructed with two of the voltagedoubler rectifying circuits 10 shown in FIG. 2, connected together by asimilar method to that of FIG. 12. This is because the lower limitthreshold value Th3AL of the input terminal A is a high level, making itnecessary to produce a feedback voltage higher than this lower limitthreshold value Th3AL. This feedback voltage is adjusted by a resistorR60 to be set the level to within the threshold value range of the inputterminal A.

With the self hold circuit 30, when a logic value of F2=1 is generatedwith switching OFF the solenoid, and a logic value of F3=2 issimultaneously generated if the residual pressure sensor is normal, thenan alternating current output signal is produced from the windowcomparator WC3. This output signal is rectified by the rectifyingcircuit 31 and fed back via the resistor R60 to the input terminal A ata level within the threshold value range thereof. Therefore, even if thesignal F3 input to the input terminal A becomes a logic value of F3<2(subsequent to time T2 of FIG. 13), then as long as a logic value ofF2=1 is being input to the input terminal B, an output is produced andself held. Furthermore the self hold circuit 30 has a fail-safeconstruction since in a worst case scenario with a fault in the windowcomparator WC3, a disconnection fault in the feedback resistor R60, anda fault in the rectifying circuit 31, then either the alternatingcurrent output from the window comparator WC3 is not produced, or theself hold function is lost.

Such a fail-safe self hold circuit wherein the oscillating output signalis rectified and fed back to the input terminal is disclosed in U.S.Pat. 5,027,114. Having the trigger condition for the self hold circuit30 as F3=2, is functionally the same as having the self hold circuit 30triggered with input terminal A having the logical product of the signalF1 and the signal F2.

A rectifying circuit 32 has a similar construction to the voltagedoubler rectifying circuit 10 of FIG. 2, giving an output of logicaloutput F4=1 with the alternating current output signal of the windowcomparator WC3 clamped at the power source voltage Vcc. A windowcomparator WC4 serving as the third logical product operating device,has the logical output F4 from the rectifying circuit 32 input to oneinput terminal A and the logical output F1 from the residual pressuresensor input to the other input terminal B, carrying out a logicalproduct operation on both to generate an alternating current outputsignal when both logical outputs F4 and F1 are "1" (F4=1, F1=1). Arectifying circuit 33 clamps the alternating current output signal ofthe window comparator WC4 at the power source voltage Vcc and rectifies,outputting the logical product output of the logical outputs F4 and F1as F5.

With the circuit of FIG. 15, after the solenoid valve is switched OFFand the residual pressure sensor detects the residual pressure condition(giving F1=0), then until a sufficient drop in the residual pressure isdetected (giving F1=1), F5=1 is not generated. In a worst case scenariowhere a blockage occurs during an established pressurised condition, anda fault occurs in another circuit such as that of the current sensor 22,the residual pressure sensor, or the rectifying circuit, then a logicaloutput F5=1 indicating no residual pressure is not produced.

With the construction of a third embodiment as shown in FIG. 17, theoutput F2 from the current sensor 22 is input to a set input of acounter 41, and the output F1 from the residual pressure sensor is inputto a reset input. The timing of a clock signal from a clock generatingcircuit 42 is started with the input of F2=1 from the current sensor 22indicating that the solenoid valve is OFF, and is stopped with the inputof F1=1 from the residual pressure sensor indicating no residualpressure, and the timing value output from the counter 41 at that timeis then displayed. In this way, the time from switching off the solenoidvalve until generation of an output for no residual pressure can bedisplayed. Hence, if the time from switching off the solenoid valveuntil generation of an output for no residual pressure is too long, thenit can be known that operation of the pressure sensing pipe 5 has becomesluggish due for example to deterioration.

With the construction wherein as shown in FIG. 12, the logical output F3from the circuit of FIG. 11 is used for displaying the operatingcondition of the sensor, since in the event of a fault in the residualpressure sensor, F1=0 is generated, then there is the problem that ifthe logical output becomes F3≧1, there will be no display for a fault inthe residual pressure sensor.

FIG. 18 shows a circuit of a fourth embodiment to cope with thisproblem.

In FIG. 18, first and second window comparators WC5, WC6 have thefail-safe circuit construction as shown in FIG. 3. The window comparatorWC5, has the logical output F3 from the voltage doubler rectifyingcircuit 27 of FIG. 11 input commonly to input terminals A and B. Thewindow comparator WC6, has an output from the window comparator WC5which has been rectified by a voltage doubler rectifying circuit 51(giving a logical output F7), input commonly to input terminals A and B.An output from the window comparator WC6 is rectified by a voltagedoubler rectifying circuit 52, and the rectified output is output as alogical output F8.

The voltage doubler rectifying circuit 51 which comprises two capacitorsC15, C16 and two diodes D15, D16, clamps the output from the windowcomparator WC5 at the power source voltage Vcc, and generates an outputF7. However, since the electrostatic capacity of the smoothing capacitorC16 is set considerably larger than that of the coupling capacitor C15,the falling response of the output F7 is slowed down. That is to say,the voltage doubler rectifying circuit 51 has an off-delay function andcorresponds to the off-delay device.

The voltage doubler rectifying circuit 52 is the same as the voltagedoubler rectifying circuit 10 of FIG. 2.

The operation will now be described with reference to the time chart ofFIG. 19.

First is a description of the case wherein the residual pressure sensorand the circuit of FIG. 11 are normal.

The output F3 from the circuit of FIG. 11, as shown in FIG. 19(a), isF3=1 until the solenoid valve is switched OFF at time T1. Then fromswitching OFF the solenoid valve giving F2=1 as shown in FIG. 19(c)until time T2 where an output is generated for no residual pressure inthe residual pressure sensor, the output is F3=2, and after generationof the output for no residual pressure in the residual pressure sensor,this becomes F3=1. Since as shown in FIG. 19(a), an upper limitthreshold value Th5H and a lower limit threshold value Th5L of thewindow comparator WC5 are set on either side of F3=1, then during aperiod T_(D) between time T1 and time T2, the oscillating output fromthe window comparator WC5 ceases and the output becomes a low level.Here, since the capacity of the capacitor C16 of the voltage doublerrectifying circuit 51 is set so as to have a delay time T_(N) which islonger than the period T_(D) between times T1 and T2 (T_(N) >T_(D)),then as shown in FIG. 19(b), the voltage doubler rectifying circuit 51also continuously generates a high level output signal F7 (shown as alogic value F7=1) during the period T_(D). Once the period T_(D) haselapsed, then after this the output becomes F3=1 due to the output F1=1indicating no residual pressure in the residual pressure sensor, and analternating current output signal is again generated from the windowcomparator WC5.

Accordingly, as long as the circuit of FIG. 11, and the residualpressure sensor are normal, then as shown by the full line in FIG.19(b), the output F7 from the voltage doubler rectifying circuit 51remains at F7=1. Since a lower limit threshold value Th6L of the windowcomparator WC6 is set as shown in FIG. 19(b), to a value lower than thelogic level indicated by logic value F7=1, then in this case, analternating current output is continuously produced from the windowcomparator WC6, and a high level logic value output F8=1 higher than thepower source voltage Vcc indicating a normal situation, is continuouslyproduced from the voltage doubler rectifying circuit 52.

Next is a description of a case wherein the residual pressure sensor isfaulty.

When at the time of pressurization the residual pressure sensor isfaulty, then the output from the residual pressure sensor does notbecome F1=0, and as shown by the dotted line in FIG. 19(a), F3=2continues on even after time T2. Therefore after the oscillating outputfrom the window comparator WC5 has stopped at time T1, once the delaytime T_(N) of the voltage doubler rectifying circuit 51 has elapsed, theoutput F7 from the voltage doubler rectifying circuit 51, as shown bythe dotted line of FIG. 19(b), becomes a level corresponding to logicvalue F7=0. The output from the window comparator WC6 thus ceases, andthe output from the voltage doubler rectifying circuit 52 becomes a lowlevel (power source voltage Vcc) with F8=0.

Moreover, if at the point of switching ON the solenoid valve, a blockagehas already occurred in the residual pressure sensor, then the output F1from the residual pressure sensor does not become F1=0, so that F7 =0prior to time T1, resulting in F8=0. Furthermore with the circuit ofFIG. 18, if a fault occurs in one or other of the circuits of FIG. 2 andFIG. 11, the output becomes F8=0.

If for example as shown by the dotted line in FIG. 18, a resistor R70and a display diode D10 are connected to the output side of the voltagedoubler rectifying circuit 52, such that the display diode D10 comes onwith an output of F8=1 and goes off when F8=0, then as well as beingable to warn of a fault in the residual pressure sensor, it is alsopossible to give a warning at the time of a fault in other circuits.

The circuits of FIG. 11 and FIG. 15 can also be applied to aconstruction for detecting residual pressure with a pressure switchusing a diaphragm, instead of the pressure sensing pipe (Bourdon tube).Here the construction may be such that at the time of no pressure theelectrical contacts come ON (F1=1) while at the time of pressure, theelectrical contacts go OFF (F1=0). In this case a blockage in theBourdon tube is equivalent to a puncture of the diaphragm, with thepressure switch not going OFF even though the solenoid valve is switchedON.

Moreover, with the respective embodiments, a two input fail-safe windowcomparator is used for the window comparators WC2, WC5, and WC6, whichare commonly supplied with the input signals. However a single inputwindow comparator can also obviously be used. A single input windowcomparator is disclosed for example in U.S. Pat. No. 5,027,114.

Furthermore, with regards to the blockage location, this has beendescribed as being at the pressure inlet of the residual pressuresensor. However, this can obviously be dealt with in the same manner ifit is in the pressure supply pipe between the solenoid valve and theresidual pressure sensor attachment portion.

Also, with the respective embodiments, since the lower limit thresholdvalues of the respective window comparators are higher than the powersource voltage Vcc, then the output voltage of the rectifying circuit isclamped at the power source voltage Vcc and output. However if ingeneral, a method is used wherein the alternating current signal isrectified after using transformer coupling, then by appropriatelydesigning the winding ratio of the primary and secondary windings of thetransformer, it is possible to optionally output a voltage which reachesthe threshold value of the window comparator. Therefore in this case itis not altogether necessary to clamp the rectified output signal at thepower source voltage.

With the residual pressure sensor of the present invention as describedabove, an output indicating no residual pressure is not produced untilthe residual pressure has completely gone and the movement of the closedend of the pressure sensing pipe has stopped. Therefore if applied to adangerous system wherein a residual pressure remains even though thepressure supply has stopped, then the safety of the system can beimproved.

Moreover, with the residual pressure sensor monitoring apparatus of thepresent invention, since it is possible to reliably warn of a blockagein the residual pressure sensor, or a fault in the sensor, then withsystems wherein detection of no residual pressure is carried out using aresidual pressure sensor, operator safety can be considerably improved.

INDUSTRIAL APPLICABILITY

The present invention can reliably verify that there is no residualpressure in a system using hydraulically powered machinery, as well asbeing able to warn of a fault in the residual pressure sensor. Henceoperator safety can be ensured and industrial applicability is thusconsiderable.

We claim:
 1. A residual pressure sensor incorporating a pressure sensing pipe with one end closed such that the closed end is displaced with an increase/decrease in pressure introduced from another end opening, and a pressure-electricity converter section which detects the displacement location of the closed end of the pressure sensing pipe and at the time of a pressure increase, decreases an electrical output in accordance with displacement of the closed end, and at the time of a pressure decrease, increases an electrical output in accordance with displacement of the closed end comprising;an electrical output change detection means for detecting whether or not the electrical output from said pressure-electricity converter section has a changing condition, and generating a low level output in the event of a changing condition, and a high level output in the event of a constant condition, and a fail-safe first logical product operating means for carrying out a logical product operation on an output from said electrical output change detection means and an output from said pressure-electricity converter section, and generating an output of logic value "1" corresponding to a high level indicating no residual pressure, when both outputs are at a high level equal to or above a predetermined value, and generating an output of logic value "0" corresponding to a low level, at the time of a fault.
 2. A residual pressure sensor according to claim 1, wherein said pressure-electricity converter section comprises;a plate having a slit and fixed to said pressure sensing pipe closed end so as to be displaced in accordance with displacement of said closed end, a light sensor incorporating a light emitting element and a light receiving element oppositely disposed with said plate therebetween, a first signal generator for supplying an alternating current signal to the light emitting element of said light sensor to generate an alternating current light beam, and a first rectifying circuit for clamping at a power source voltage and rectifying, an alternating current output from said light sensor, the construction being such that when a pressure in said pressure sensing pipe is equal to or less than a predetermined pressure, a light beam from said light emitting element is received by said light receiving element via said slit.
 3. A residual pressure sensor according to claim 2, wherein a vibrating element is fitted to said plate to vibrate said plate in a direction substantially perpendicular to direction of light emission from the light emitting element to modulate the light beam emitted from the light emitting element.
 4. A residual pressure sensor according to claim 1, wherein said pressure-electricity converter section comprises;a plate fixed to said pressure sensing pipe closed end so as to be displaced in accordance with displacement of said closed end, a light sensor incorporating and a light emitting element and light receiving element provided to one side of said plate, a first signal generator for supplying an alternating current signal to the light emitting element of said light sensor to generate an alternating current light beam, and a first rectifying circuit for clamping at a power source voltage and rectifying, an alternating current output from said light sensor, the construction being such that when a pressure in said pressure sensing pipe is equal to or less than a predetermined pressure, the light beam from said light emitting element is reflected by said plate and received by said light receiving element.
 5. A residual pressure sensor according to claim 1, wherein there is provided; two pressure-electricity converter sections which respectively generate electrical output signals of a high level at the time of pressure levels equal to or less than mutually different first and second pressure levels, anda first self hold circuit with an output from the pressure-electricity converter section which generates an electrical output signal at pressure levels equal to or less than the first pressure level, as a trigger input signal, and an output from the pressure-electricity converter section which generates an electrical output signal at pressure levels equal to or less than the second pressure level which is higher than the first pressure level, as a reset input signal, and which self holds the trigger input signal.
 6. A residual pressure sensor according to claim 1, wherein said electrical output change detection means comprises;a second signal generating means for superimposing a high frequency alternating current signal on an output from the pressure-electricity converter section, an amplifying means into which is input by way of a coupling capacitor, the output from the pressure-electricity converter section on which is superimposed the high frequency alternating current signal of said second signal generating means, and wherein the amplified output is saturated when the output from the pressure-electricity converter section is in a changing condition, and a second rectifying circuit for clamping the alternating current amplified output from said amplifying means at the power source voltage and rectifying, the construction being such that the rectified output from said second rectifying circuit is output to said first logical product operating means.
 7. A residual pressure sensor according to claim 1, wherein said first logical product operating means is constructed of a fail-safe window comparator having two input terminals, which generates an alternating current output higher than the power source voltage when each of the input signals input to the respective input terminals are equal to or above a previously set lower limit threshold value, and which generates an output of logic value "0" at the time of a fault.
 8. A residual pressure sensor monitoring apparatus, applicable to a system incorporating a pressure supply control means for carrying out pressure supply to a movable section of a machine at the time of electrical power supply and for stopping pressure supply at the time of no electrical power supply, which monitors that there is no pressure supply to said movable section of the machine using a residual pressure sensor incorporating a pressure sensing pipe with one end closed such that the closed end is displaced with an increase/decrease in pressure introduced from another end opening, and a pressure-electricity converter section which detects the displacement location of the closed end of the pressure sensing pipe and at the time of a pressure increase, decreases an electrical output in accordance with displacement of the closed end, and at the time of a pressure decrease, increases an electrical output in accordance with displacement of the closed end, said residual pressure sensor monitoring apparatus being for monitoring if the operation condition of said residual pressure sensor is normal or abnormal, said residual pressure sensor monitoring apparatus comprising;a current sensor which monitors the electrical power supply condition of said pressure supply control means, and generates a low level output of logic value "0" at the time of electrical power supply, and generates a high level output of logic value "1" at the time of no electrical power supply, and generates an output of logic value "0" at the time of a fault, a fail-safe NOT operating means which carries out a NOT operation on the logical output from said residual pressure sensor, which generates a low level output of logic value "0" at the time of supply pressure to the movable section of the machine, and a high level output of logic value "1" at the time of no supply pressure, and generates an output of logic value "0" at the time of a fault, and which generates a low level output of logic value "0" at the time of a fault, and judgement means for judging if there is a residual pressure sensor fault based on respective logical outputs from said current sensor and said NOT operating means, and when both logical outputs are logic value "0", generates a low level output of logic value "0" indicating a fault in the residual pressure sensor.
 9. A residual pressure sensor monitoring apparatus according to claim 8, wherein said NOT operating means comprises a fail-safe window comparator which generates a high level output of logic value "1" when the input level input to an input terminal is within a previously set upper and lower threshold value range, and generates an output of logic value "0" at the time of a fault, said upper and lower threshold values being set on either side of an output level of logic value "0" of said residual pressure sensor.
 10. A residual pressure sensor monitoring apparatus according to claim 8, wherein said judgement means is an adding circuit which carries out an addition operation on the respective logical outputs of said current sensor and said NOT operating means.
 11. A residual pressure sensor monitoring apparatus according to claim 10, wherein said adding circuit is a rectifying circuit which clamps an output from said NOT operating means at an output level of said current sensor, and rectifies.
 12. A residual pressure sensor monitoring apparatus according to claim 8, wherein said current sensor comprises,a saturable magnetic body core wound with three windings, namely first second and third windings which becomes saturated when a drive current to said pressure supply control means flows in said third winding, a second signal generator for supplying a high frequency signal to said first winding, an AC amplifier connected to said second winding for amplifying a signal received by said second winding, and a third rectifying circuit for clamping an output from said AC amplifier at the power source voltage, and rectifying and outputting.
 13. A residual pressure sensor monitoring apparatus according to claim 8, wherein there is provided a second logical product operating means for carrying out a logical product operation on the output from said judgement means and the output from said current sensor, and a third logical product operating means for carrying out a logical product operation on the output from said second logical product operating means and the output from said residual pressure sensor, and the output from said third logical product operating means is made the residual pressure sensor fault judgement output.
 14. A residual pressure sensor monitoring apparatus according to claim 13, wherein when said judgement means is an adding circuit, said second logical product operating means comprises a fail-safe self hold circuit which self holds with the output from said current sensor as a reset input and the output from said adding circuit as a trigger input, and which generates an output of logic value "0" at the time of a fault.
 15. A residual pressure sensor monitoring apparatus according to claim 8, wherein said judgement means is an adding circuit, and there is provided;a fail-safe first window comparator with upper and lower threshold values set on either side of an intermediate value of an addition output from said adding circuit, which generates a high level output of logic value "1" when an addition output within the threshold value range is input, and which generates an output of logic value "0" at the time of a fault, an off-delay means which time delays a drop in the output from said first window comparator to longer than a period from when the current sensor generates an output indicating no current until the residual pressure sensor generates an output indicating no residual pressure, and a second window comparator which generates an output of logic value "0" indicating a residual pressure sensor fault when the output from said off-delay means is lower than a predetermined level.
 16. A residual pressure sensor monitoring apparatus according to claim 8, wherein a counter is provided which counts a clock signal at the time of inputting an output from the current sensor of a logic value of "1" indicating no current, and stops counting at the time of inputting an output from the residual pressure sensor of logic value "1" indicating no pressure.
 17. A residual pressure sensor monitoring apparatus according to claim 8, wherein said residual pressure sensor comprises;an electrical output change detection means for detecting whether or not the electrical output from said pressure-electricity converter section has a changing condition, and generating a low level output in the event of a changing condition, and a high level output in the event of a constant condition, and a fail-safe first logical product operating means for carrying out a logical product operation on an output from said electrical output change detection means and an output from said pressure-electricity converter section, and generating an output of logic value "1" corresponding to a high level indicating no residual pressure, when both outputs are at a high level equal to or above a predetermined value, and generating an output of logic value "0" corresponding to a low level, at the time of a fault.
 18. A residual pressure sensor monitoring apparatus according to claim 17, wherein said pressure-electricity converter section comprises;a plate having a slit and fixed to said pressure sensing pipe closed end so as to be displaced in accordance with displacement of said closed end, a light sensor incorporating and a light emitting element and a light receiving element oppositely disposed with said plate therebetween, a first signal generator for supplying an alternating current signal to the light emitting element of said light sensor to generate an alternating current light beam, and a first rectifying circuit for clamping at a power source voltage and rectifying, an alternating current output from said light sensor, the construction being such that when a pressure in said pressure sensing pipe is equal to or less than a predetermined pressure, a light beam from said light emitting element is received by said light receiving element via said slit.
 19. A residual pressure sensor monitoring apparatus according to claim 18, wherein a vibrating element is fitted to said plate to vibrate said plate in a direction substantially perpendicular to direction of light emission from the light emitting element to modulate the light beam emitted from the light emitting element.
 20. A residual pressure sensor monitoring apparatus according to claim 17, wherein said pressure-electricity converter section comprises;a plate fixed to said pressure sensing pipe closed end so as to be displaced in accordance with displacement of said closed end, a light sensor incorporating and a light emitting element and light receiving element provided to one side of said plate, a first signal generator for supplying an alternating current signal to the light emitting element of said light sensor to generate an alternating current light beam, and a first rectifying circuit for clamping at a power source voltage and rectifying, an alternating current output from said light sensor, the construction being such that when a pressure in said pressure sensing pipe is equal to or less than a predetermined pressure, the light beam from said light emitting element is reflected by said plate and received by said light receiving element.
 21. A residual pressure sensor monitoring apparatus according to claim 17, wherein there is provided;two pressure-electricity converter sections which respectively generate electrical output signals of a high level at the time of pressure levels equal to or less than mutually different first and second pressure levels, and a first self hold circuit with an output from the pressure-electricity converter section which generates an electrical output signal at pressure levels equal to or less than the first pressure level, as a trigger input signal, and an output from the pressure-electricity converter section which generates an electrical output signal at pressure levels equal to or less than the second pressure level which is higher than the first pressure level, as a reset input signal, and which self holds the trigger input signal.
 22. A residual pressure sensor monitoring apparatus according to claim 17, wherein said electrical output change detection means comprises;a second signal generating means for superimposing a high frequency alternating current signal on an output from the pressure-electricity converter section, an amplifying means into which is input by way of a coupling capacitor, the output from the pressure-electricity converter section on which is superimposed the high frequency alternating current signal of said second signal generating means, and wherein the amplified output is saturated when the output from the pressure-electricity converter section is in a changing condition, and a second rectifying circuit for clamping the alternating current amplified output from said amplifying means at the power source voltage and rectifying, the construction being such that the rectified output from said second rectifying circuit is output to said first logical product operating means.
 23. A residual pressure sensor monitoring apparatus according to claim 17, wherein said first logical product operating means is constructed of a fail-safe window comparator having two input terminals, which generates an alternating current output higher than the power source voltage when each of the input signals input to the respective input terminals are equal to or above a previously set lower limit threshold value, and which generates an output of logic value "0" at the time of a fault. 